KELATIONSIIIP OF PlI YTOI'LAXKTOX TO LIMNOLOGY 



67 



of factors which, taken together, are able to 

 determine the type of flora and fauna. At 

 the same time we realize that the many 

 djmamic factors in operation result in an 

 ever changing set of limiting factors which 

 thus make possible variations in the fauna 

 and flora from year to year. Nevertheless, 

 a lake may have a combination of character- 

 istics in common with other bodies of water 

 and when such is the case we might Avell 

 exjDCct to find somewhat, if not quite, simi- 

 lar floras. Thus, when enough lakes have 

 been studied we should expect to find asso- 

 ciations of organisms representative of those 

 lakes which have similar environmental fac- 

 tors. This we find in the classifications of 

 Thienemann, and of Naumann; classifica- 

 tions which have proved helpful, particu- 

 larh' for an understanding of European 

 lakes. 



In North America we find representatives 

 of the oligotrophic, eutrophic, and dys- 

 trophic tyi^es. The deep oligotrophic lakes 

 are characterized by a Chlorophycean flora, 

 low in quantity, but often high in number 

 of species, and with a conspicuous Desmid 

 element (Fig. 1). 



The shallower, eutrophic lakes are domi- 

 nated bv Mvxophvcean and diatom floras 

 and, in certain regions of North America, 



tlicy ;ii-e often blanketed with "water- 

 bloom" during the warm season. 



The very shallow dystrophic type, of un- 

 certain definitions, supports dense beds of 

 vegetation. The phytoplankton is hetero- 

 geneous, although poor in quantity, and 

 mats of filamentous Chlorophyceae are 

 usually present (Fig. 1). 



While these types of floras generally are 

 to be associated with the recognized limno- 

 logical types, we find, as is well known, that 

 very many lakes do not conform, or do so 

 in part only, and invite, therefore, analyti- 

 cal studies. A review of published analyses 

 of lakes in this country and my own obser- 

 vations, lead me to submit, reservedly, the 

 prediction that the time must soon come, if 

 indeed it has not already arrived, when we 

 can and should examine data at hand with 

 the view of drawing up a limnological and 

 ]:)hytoplankton classification of North Amer- 

 ican lakes which will helj) us to discuss them 

 with more standardized and accurate dis- 

 tinctions. I make this prediction in spite 

 of the fact that some students are pessimis- 

 tic over the possibility of ever being able to 

 classify lakes according to their phyto- 

 plankton types. 



We have, for example, certain critically 

 studied bog lakes, the phytoplankton and 



Fig. 2. A bog lake, surrounded by a mat of Sphagnum and beds of Cliamacdaphne. 



